The present invention relates to a liquid crystal display using a component for liquid crystal displays, such as rear illumination equipment.
In recent years, the implementation of personal computers, inclusive of so-called word processors, in a small size has been promoted, and portable type personal computers, known as lap-top type or notebook type computers, are now widely used. In such a portable type personal computer, a liquid crystal device is commonly used as a display unit. In this regard, there is an increasing tendency for adopting a color display in portable type personal computers. In line with such a trend, a backlighting type display device is coming into wide use, in which a light source is disposed at a rear side of a liquid crystal display screen for lighting the whole display screen from the rear or back side. Needless to say, the backlighting light source for the color liquid crystal display device is required to emit light with high luminance. Besides, it is necessary to illuminate the display screen with uniform luminance over the whole planar surface thereof. Luminance of the backlighting can easily be increased by increasing the luminance of the light source. However, taking into consideration the fact that a portable-type personal computer or word processor or the like is usually operated br using a battery or storage cell, a limitation concerning the voltage supply is necessarily imposed on any attempt to increase the luminance of the light source. Stated in another way, no other effective method or measures for increasing the luminance of the liquid crystal display screen have been proposed to date.
For having better understanding of the present invention, a description will first be made in some detail of conventional liquid crystal display devices, such as disclosed, for example, in JP-A-4-162002 and JP-A-6-67004. FIG. 2 shows a lateral source type backlighting device employed conventionally in a liquid crystal display device known heretofore. Referring to the FIG. 2, a lamp, such as a cold-cathode discharge tube or a hot-cathode discharge tube, is employed as a light source 1 which is disposed at and along one lateral side of a light guide plate (also known as optical waveguide plate) 2 which is made of a light-transmissive material. Here, an optical scattering layer 3 from which light is scattered and a reflection sheet 4 that causes light to reflect are provided on the underneath side of said light guide plate 2. And, a diffusion sheet 5, that consists of synthetic resins of milk-white color that have an optical scattering effect, is provided in the area over the surface of said light guide plate 2 to pass and to illuminate the whole face with a uniform brightness. In addition, a first condensation sheet 6 to converge diffused light to some extent on the face and to improve the brightness of the front face of the display and a second condensation sheet 7 are arranged above the diffusion sheet 5. As to the optical scattering layer 3, which is shown in more detail in FIG. 3, it consists of a plurality of ink dots 8, formed of optical scattering materials, such as oxide titanium, arranged on the surface of light guide plate 2. As the distance increases from the light source 1, the optical intensity from light source 1 is reduced. Therefore, as the distance increases from the light source 1, as shown in FIG. 3, the area of the ink dots 8 is increased.
As described above, there is a problem of the brightness declining due to the loss of optical scattering in conventional illumination equipment. The reason is because light is emitted from light source 1, conducted to light guide plate 2, scattered by optical scattering material 8 that is contained in the optical scattering layer, passes through a diffusion board later, and then irradiates a liquid crystal element.
There is a light guide plate not using ink dots, to solve this problem, as described in JP-A-9-269489. This light guide plate has a small convex or a small concave area formed on the surface thereof. These small convex or small concave areas reflect light, and a liquid crystal element is illuminated thereby. But the shape and distribution of these areas were not optimized, and so there was still room for a further improvement in brightness.
In addition, as one of the narrow advances that led to an improvement in the brightness of a liquid crystal display, it has been proposed that the permeative rate of a polarization filter should be raised. The polarization filter is an element that is arranged between a liquid crystal cell and the back light and which has the function of passing only a specified polarization light into a liquid cell. The polarization filter can be manufactured by adsorbing a dichroic material in the micell pipe of a macromolecule film that generally arranges a micell in a constant direction. As a macromolecule film, polyvinyl alcohol is used. Between rollers on which this polyvinyl alcohol spins at a different speed, it is drawn about 3-5 times in the constant direction. The micell of a drawn PVA (polyvinyl alcohol) is arranged in the drawing direction, and the arranged film has a strong double refraction. There are halogen materials, such as a iodine and a dyestuff, as a material to give dichroism. By adsorbing the above material in the film being drawn, polarization characteristics are expressed. As for the above polarization filter, a polarization separation function is gained easily. But the permeative rate is small, being 50% or less. Because a dichroic material is used theoretically, polarization light that is orthogonal with the polarization light that is transmitted is absorbed. Therefore, it is a present condition that 50% or more of the optical energy is lost by a polarization filter, and the brightness of a liquid crystal display element using such a filter is remarkably reduced as a result.
It has been proposed to adopt a method of using a polarizability film of a reflection type as a means of obtaining improved brightness. A polarizability film of a reflection type is a film that has a property such that all polarization components other than a polarization component of a specific kind are reflected and only the specific polarization component is passed, like a cholesteric liquid crystal film, etc. That is, the polarizing natural light that comes out through a light guide plate is applied to a polarizability film of a reflection type, whereby only a specific polarization component is transmitted, and all other polarization components are reflected. A reflected polarization component is reflected again by a reflection board later, the polarization state is changed, it is applied to the polarizability film of a reflection type again, and only the specific polarization component is allowed to pass through. By repeated reflections, all components of light can be used.
A method of using a cholesteric liquid crystal film representing a polarizability film of a reflection type has been proposed in JP-A-3-45906 and the JP-A-6-281814. The cholesteric liquid crystal film consists of optical active layers of a polymer material that has cholesteric regularity. The cholesteric liquid crystal film transmits only a circular polarization component of the same direction as the spiral direction of a cholesteric layer in the polarizing natural light that comes out through a light guide plate from the light source, and the circular polarization component of a reverse direction is reflected. Therefore, when it has the structure shown in FIG. 16, a reflected circular polarization component is reflected again with a reflection sheet, it is returned, it returns it to a state that is close to natural light, and the cholesteric liquid crystal film is entered again. By a repeat of this cycle, all components of light can be used. When a xc2xc phase sheet is formed on the surface at which light appears on this cholesteric liquid crystal film, a circular polarization component is converted into a straight line polarization, and so the arrangement can be used as rear luminescence equipment for a liquid crystal display.
The brightness of a liquid crystal display using a cholesteric liquid crystal film becomes double the brightness of the liquid crystal display which uses an ordinary polarization filter from theoretical viewpoint. But, in case a light guide plate of the ink dot type, that is widely used at present, and a cholesteric liquid crystal film are combined, the results are unsatisfactory. The reason is because it incurs a loss due to the scattering mentioned above, and when the light which does not pass through the cholesteric liquid crystal film is reflected, the circular polarization that is injected into the light guide plate again is reflected (diffusion) by an ink dot, and loss by scattering occurs, and the polarization state also is degraded further. And, as for the dot, the size is quite large, and so it is necessary to use a diffusion sheet in combination to prevent dot visibility, with the result that the brightness improvement effect declines further. In case it is combined with a prism sheet that optimizes the angle of distribution of the light that comes out and that improves front face brightness, the brightness improvement effect is not more than 10%.
However, a method of combining the light guide plate that is formed with a grating of grooves in the surface thereof and a cholesteric liquid crystal film has been proposed in JP-A-9-102209. This combination produces high brightness, because it has a small loss by scattering, and comparing the brightness improvement effect with that of a printing dot, it is high.
But as for the light guide plate that is formed with grating grooves, it is difficult to control the brightness distribution in the X direction, and it is expected that problems with the cost of metal mold manufacture and with meeting an appointed date of delivery is also large. In addition, there is a fear of a low mass production. The reason is because, from the point of view of surface roughness, soft metals, such as brass, must be used as a metal mold material. In addition, it is expected that moire will easily occur between periodically formed grating grooves and a liquid crystal cell. And, as for the grating grooves, the period is quite large, it is necessary to use a diffusion board in combination, and so the brightness improvement effect has a tendency to decline further. In case it is combined with a prism sheet that optimizes the angular distribution of the light that comes out and that improves the front face brightness, its brightness improvement effect is not more than 20%.
To irradiate the liquid crystal element side with light from a light source using scattering provided by an ink dot with conventional equipment such as that described above, light is absorbed at the time of being scattered by the ink dot, and so there is a limit to the improvement in efficiency available with such a system. And, with regard to a light guide plate not using ink dots, the shape and distribution of the light are not optimized, and so there is room for an improvement in brightness. In the combination of a light guide plate using the ink dot method and a cholesteric liquid crystal film, the brightness improvement effect is as low as 10%. While the combination of the light guide plate having grating grooves and a cholesteric liquid crystal film, the brightness improvement effect is only as high as 20%, and so it is difficult to control the brightness distribution and to economically manufacture the light guide plate.
To solve the problems inherent in conventional devices, the present invention is proposed.
It is an object of the present invention to provide a liquid crystal display that can improve brightness without enhancing conventional faults and without increasing the brightness of the light source.
In this regard, by using a light guide plate that has formed therein several small concaves (called dots in the following description) to convert the direction of light waves in the light guide plate from a specified direction toward the liquid crystal display, and by properly designing the plane shape and a section inclination angle of the dots, are the objects of the present invention can be achieved.
A reflection sheet is arranged underneath the light guide plate. In addition, illumination light that has a proper angular distribution from the light emitting face to the prism sheet that has a proper prism vertical angle according to a requirement can be used to irradiate a display element.
In addition, the dots are arranged at random to prevent moire from occurring. And, light comes out of a dot with a specified density distribution, and so equalization of the brightness distribution of light is achieved.
In addition, between the light emitting surface and liquid crystal unit, a polarizability film of a reflection type (cholesteric liquid crystal film, xc2xc phase sheet, etc) is arranged, with a result that brightness is improved.